In this paper we experimentally demonstrate a transmission delay compensation scheme for precise fiber-optic time transfer. The scheme is based on a clock counter and an electronic variable delay line, which theoretically can provide unlimited compensation range. We perform successive tests in three optical fiber links of different lengths in which both continuous drifts and abrupt hop of the transmission delay are effectively compensated. The total transmission delay variation induced in the experiments is much larger than most of the reported cases. This large-dynamic compensation scheme is quite suitable for time transfer links whose transmission delay varies a lot.
The microbial community structure of sediments in the Bahe River Basin, China was studied using a high-throughput sequencing platform and PCR amplification to investigate the pattern changes in microbial communities in urban rivers affected by anthropogenic activities and their environmental driving mechanisms. The results demonstrated that the average total nitrogen and total phosphorus in the sediments were 524 and 734 mg/kg, respectively. The T, COD and $${\text{NH}}_{4}^{ + }$$
NH
4
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-N of the water and the moisture content of the sediments has significantly impacted on the microbial community structure. Twenty microbial species with a relative abundance > 1% in the sediments of the river were observed, accounting for 95–99% of the total microbial community. The primary species were Proteobacteria (13.86–69.14%), Firmicutes (1.45–58.33%), Chloroflexi (3.68–26.18%), Actinobacteria (2.7–21.51%), Acidobacteria (0.73–16.36%), Bacteroides (1.53–14.11%), and Thermodesulfobacteria (0.1–8.9%), accounting for over 90% of the total microbial community. At the class level, the primary species were γ-proteobacteria, Alphaproteobacteria, Anaerolineae, Bacillus, Bacteroidota, Actinobacteriota, and Clostridia, accounting for over 70% of the total microbial community. Our results provide direct evidence for a link between microbial community structure and environment factors. This evidence demonstrates that sediment microorganisms can be applied to evaluate urban rivers pollution levels, which can provide a scientific basis for pollution control and management in the urban river affected by human activities.
In order to detect weak light from background radiation, utilizing an array of Geiger-mode avalanche photodiodes (GM-APDs) as a detector in the receiver is researched. First, the principle of a GM-APD operated in gated mode is analyzed. Based on the Poisson random process of arrival of photons, the trigger probability of a single GM-APD has been computed. Then, according to the trigger probability, a new bit error rate (BER) model of a photon-counting array receiver system is built, and the expression of BER is derived based on the quantum efficiency, signal photons, background photons, dark carriers, and the number of GM-APDs in array. Using the BER expression, the error performance of a new photon-counting array receiver system can be predicted rapidly. Furthermore, the error performance of a system based on an array of GM-APDs is investigated. The simulation results indicated that maintaining the ratio of signal photons and background photons above 1 is necessary for system operation. When background noise is limited to the acceptable range, maintaining a certain scale of GM-APDs (above 20) in array can greatly reduce the demand of transmitting power. Compared with achieving target BERs of 10 and 10, preserving a BER of 10 and making use of error correction coding can further improve the receiving sensitivity to 5-8 dB.
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